As a result of the interest in recent years to reduce the emission of pollutants from burners used in large furnaces, burner design has undergone substantial change. In the past, improvements in burner design were aimed primarily at improving heat distribution. Increasingly stringent environmental regulations have shifted the focus of burner design to the minimization of regulated pollutants.
Oxides of nitrogen (NOx) are formed in air at high temperatures. These compounds include, but are not limited to, nitrogen oxide and nitrogen dioxide. Reduction of NOx emissions is a desired goal to decrease air pollution and meet government regulations. In recent years, a wide variety of mobile and stationary sources of NOx emissions have come under increased scrutiny and regulation.
A strategy for achieving lower NOx emission levels is to install a NOx reduction catalyst to treat the furnace exhaust stream. This strategy, known as Selective Catalytic Reduction (SCR), is very costly and, although it can be effective in meeting more stringent regulations, represents a less desirable alternative to improvements in burner design.
Burners used in large industrial furnaces may use either liquid fuel or gas. Liquid fuel burners mix the fuel with steam prior to combustion to atomize the fuel to enable more complete combustion, and combustion air is mixed with the fuel at the zone of combustion.
Gas fired burners can be classified as either premix or raw gas, depending on the method used to combine the air and fuel. They also differ in configuration and the type of burner tip used.
Raw gas burners inject fuel directly into the air stream, and the mixing of fuel and air occurs simultaneously with combustion. Since airflow does not change appreciably with fuel flow, the air register settings of natural draft burners must be changed after firing rate changes. Therefore, frequent adjustment may be necessary, as explained in detail in U.S. Pat. No. 4,257,763, which patent is incorporated herein by reference. In addition, many raw gas burners produce luminous flames.
Premix burners mix some or all of the fuel with some or all of the combustion air prior to combustion. Since premixing is accomplished by using the energy present in the fuel stream, airflow is largely proportional to fuel flow. As a result, therefore, less frequent adjustment is required. Premixing the fuel and air also facilitates the achievement of the desired flame characteristics. Due to these properties, premix burners are often compatible with various steam cracking furnace configurations.
Floor-fired premix burners are used in many steam crackers and steam reformers primarily because of their ability to produce a relatively uniform heat distribution profile in the tall radiant sections of these furnaces. Flames are non-luminous, permitting tube metal temperatures to be readily monitored. Therefore, a premix burner is the burner of choice for such furnaces. Premix burners can also be designed for special heat distribution profiles or flame shapes required in other types of furnaces.
In gas fired industrial furnaces NOx is formed by the oxidation of nitrogen drawn into the burner with the combustion air stream. The formation of NOx is widely believed to occur primarily in regions of the flame where there exist both high temperatures and an abundance of oxygen. Since ethylene furnaces are amongst the highest temperature furnaces used in the hydrocarbon processing industry, the natural tendency of burners in these furnaces is to produce high levels of NOx emissions.
One technique for reducing NOx that has become widely accepted in industry is known as staging. With staging, the primary flame zone is deficient in either air (fuel-rich) or fuel (fuel-lean). The balance of the air or fuel is injected into the burner in a secondary flame zone or elsewhere in the combustion chamber. As is well known, a fuel-rich or fuel-lean combustion zone is less conducive to NOx formation than an air-fuel ratio closer to stoichiometry. Staging results in reducing peak temperatures in the primary flame zone and has been found to alter combustion speed in a way that reduces NOx. Since NOx formation is exponentially dependent on gas temperature, even small reductions in peak flame temperature can dramatically reduce NOx emissions. However this must be balanced with the fact that radiant heat transfer decreases with reduced flame temperature, while CO emissions, an indication of incomplete combustion, may actually increase as well.
In the context of premix burners, the term primary air refers to the air premixed with the fuel; secondary, and in some cases tertiary, air refers to the balance of the air required for proper combustion. In raw gas burners, primary air is the air that is more closely associated with the fuel; secondary and tertiary air are more remotely associated with the fuel. The upper limit of flammability refers to the mixture containing the maximum fuel concentration (fuel-rich) through which a flame can propagate.
Thus, one set of techniques achieves lower flame temperatures by using staged-air or staged-fuel burners to lower flame temperatures by carrying out the initial combustion at far from stoichiometric conditions (either fuel-rich or air-rich) and adding the remaining air or fuel only after the flame has radiated some heat away to the fluid being heated in the furnace.
Another set of techniques achieves lower flame temperatures by diluting the fuel-air mixture with inert material. Flue-gas (the products of the combustion reaction) or steam are commonly used diluents. Such burners are classified as FGR (flue-gas-recirculation) or steam-injected, respectively.
U.S. Pat. No. 5,092,761 discloses a method and apparatus for reducing NOx emissions from premix burners by recirculating flue gas. Flue gas is drawn from the furnace through a pipe or pipes by the inspirating effect of fuel gas. The flue gas mixes with combustion air in a primary air chamber prior to combustion to dilute the concentration of O2 in the combustion air, which lowers flame temperature and thereby reduces NOx emissions. While burners of this type offer the advantage of supplying recycled flue gas without the need of an external fan, saving the expense of the fan and its energy consumption, flue gas drawn from the burner's radiant box is generally in the range of 1000 to 2000° F., while flue gas drawn from the furnace exhaust is generally less than 700° F. Flue gas drawn from a furnace exhaust also offers the advantage of having lower O2 content (about 1-3% vs. 5-15%), enhancing its ability to serve as a diluent.
In certain premix burners, including those disclosed in U.S. Pat. No. 5,092,761, a centering plate is utilized to assure maximum entrainment. The contents of U.S. Pat. No. 5,092,761 are incorporated herein by reference.
Analysis of burners of the type described in U.S. Pat. No. 5,092,761 has indicated the flue-gas-recirculation (FGR) ratio is generally in the range 5-10% where FGR ratio is defined as:                FGR ratio (%)=100[G/(F+A)]                    where G=Flue-gas drawn into venturi, (lb)                            F=Fuel combusted in burner, (lb), and                A=Air drawn into burner, (lb).                                                
The ability of these burners to generate higher FGR ratios is limited by the inspirating capacity of the gas spud/venturi/FGR flow ducting combination. Further closing of the primary air dampers will produce lower pressures in the primary air chamber and thus enable increased FGR ratios. However, the flow of primary air may be reduced such that insufficient oxygen exists in the venturi for acceptable burner stability.
European Application No. 1,096,202 A1 proposes fuel dilution methods and an apparatus aimed at NOx reduction. Proposed in this application is a method which includes providing a chamber outside of the burner and furnace for mixing flue gases from the furnace with fuel gas, discharging the fuel gas in the form of a fuel jet into the mixing chamber so that flue gases from the furnace are drawn into the chamber and mixed with and dilute the fuel gas therein and conducting the resulting mixture of flue gases and fuel gas to the burner wherein the mixture is combined with the combustion air and burned in the furnace. The flue gas is shown to be drawn from a furnace exhaust duct.
Disadvantages associated with the proposal of European Application No. 1,096,202 A1 include the fact that fuel gas is introduced outside of the burner and furnace in a separate chamber, an arrangement that could pose leakage problems. In addition, it is a less than an optimal arrangement to pass a potentially explosive mixture through an external duct associated with the furnace. Another disadvantage is the apparent difficulty of retrofitting the proposal to existing burners. Finally, while steam is used in connection with the teachings of European Application No. 1,096,202 A1, it is only used for the purpose of reducing NOx and not for increasing the available motive force to transport flue gas into the burner.
Despite these advances in the art, a need exists for a burner capable of utilizing flue gas for recirculating, drawn from a furnace exhaust stack, which does not require, or minimizes the need for, the use of an external fan.
Therefore, what is needed is a burner for the combustion of fuel gas and air, which better enables the use of cooler, lower oxygen content, flue gas for recirculating, permitting higher FGR recirculation ratios to be utilized, yielding further reductions in NOx emissions.